The present invention relates to a vehicle braking systems, and in particular to an apparatus and method for preventing complete immobilization of a vehicle in the event of a failure in a parking brake circuit at a wheel end.
Commercial vehicle are typically equipped with spring-actuated/fluid released parking brake systems, in particular pneumatic systems which utilize compressed air to release the parking brake actuator of each wheel end's spring brake actuator. The parking brakes in such a vehicle remain released as long as the parking brake release pressure is maintained in spring brake actuators. Such systems typically have a common source of pressure connected to each wheel end's spring brake actuator, such that application of pressure to the common lines simultaneously releases the vehicle's parking brakes.
A single point of failure in such systems may result in application of all of the wheel end spring brake actuators' parking brake actuators, effectively immobilizing the vehicle. For example, in a typical 6×4 vehicle, a rupture of any one of the four pneumatic lines to the wheel ends could result in parking brake release pressure being bled off from all four spring brake actuators. This has been the standard industry design practice, as a failure of a single brake actuator or the pressure delivery to a single axle has been viewed as a serious event warranting automatic full application of all of the vehicle's brakes as desirable “fail safe” outcome, particularly where dragging a locked wheel or an applied brake in an on-highway situation for an extended period might result in a thermal event.
Notwithstanding the common industry design practice, there may also be situations in which full parking brake application and the resulting complete immobilization of the vehicle could have undesired consequences. For example, in a commercial application of a vehicle with multiple axles such as a logging truck, being forced to park or automatically immobilize the vehicle in the event of a parking brake release pressure failure at one wheel end may be highly undesirable when the vehicle is at a very remote location (such as deep in a forest on a logging road), where repair facilities are far away and/or the immobilization of the vehicle may block a much-needed passage, such as a fire road in the case of a logging truck or the travel lanes of a high speed, high vehicle volume highway in the case of an over-the-road commercial.
Other examples where automatic application of all spring brake parking brakes in the event of a single wheel end failure may be undesirable include heavy police vehicles or military vehicles which may be subjected to combat conditions such as criminal attacks or exposure to improvised explosive devices (“IEDs”). During such an event, damage to a pneumatic line serving one wheel end in a prior art common-supply parking brake system could cause all of the wheel ends' parking brakes to be immediately applied, effectively immobilizing the vehicle in a location where the vehicle and its personnel would be vulnerable to further attacks.
It therefore would be desirable to have a parking brake system which, in addition to permitting full parking brake release and application capability in normal operating conditions, also responds to a line failure at one wheel end in a manner which provides for continued reliable release and application of the parking brakes of the remaining undamaged wheel ends.
One of the design constraints which has inhibited development of such a parking brake system is U.S. Federal Motor Vehicle Safety Standard (“FMVSS”) section 121, which requires the parking brake system to be capable of being applied within a very short period (less that 3.0 seconds from initial movement of the control device to less than 3 psi in the spring brake chamber, per the current FMVSS 121 regulation). In addition, if there is a loss of primary air brake circuit, all of the spring brakes are used to still meet the stopping distance requirement in the FMVSS and assist in keeping the vehicle in the lane. Additional valves in the delivery path to the spring brake actuator affect the spring brake modulation (i.e., the responsiveness of the spring brake actuator). Due to these time, pressure and distance requirements, those of skill in the art have sought to avoid placing additional devices in the pressure supply lines, as such intermediate components are associated with system time-response performance degradation.
The present invention solves these and other problems of the prior art by arranging devices at each wheel end which, while not inhibiting flow to such an extent that the FMVSS 121 3.0 second brake application limit is exceeded, function as wheel end shut-off devices to isolate the damaged wheel end from the remaining portions of the parking brake system while not restricting flow during spring brake modulation.
In one embodiment of the present invention, a pilot-operated, non-graduated pneumatic control valve, for example, a Bendix SV-1 Synchro Valve, available from Bendix Commercial Vehicle Systems LLC of Elyria, Ohio, is placed in the pneumatic supply line to a wheel end, between the wheel end's spring brake actuator and an upstream pressure modulator, with the pilot-operated, non-graduated pneumatic control valve's pilot control input and pressure supply input being connected in parallel (known in the art as “common supply”) to effect automatic operation of the control valve.
With this arrangement, if the supply line between the discharge port of the valve and the wheel end's spring brake actuator is breached, it has been determined in testing that the sudden decrease in pressure in the line upstream of the breach (the pressure drop resulting in from the sudden high rate of flow out of the breached line) will result in the pressure at the pilot control port dropping low enough to cause the control valve's internal piston stem to retract, automatically shutting off the flow of pressurized air from the upstream pressure modulator to the control valve's discharge port and venting the control valve through its exhaust port to atmosphere. It was also determined that the addition of the pilot-operated, non-graduated pneumatic control valve did not degrade system response times enough to prevent compliance with FMVSS 121. A further advantage of the present invention is that it may be implemented at minimal cost, using existing components re-purposed and appropriately adjusted and/or calibrated (for example, adjustment of the pilot pressure shut-off set point) for use in.
Thus, with the present invention if the damage at one wheel end is not severe enough to by itself prevent motion of the vehicle, the prevention of automatic application of the remaining parking brakes may allow the vehicle to be moved away from its present position. For example, in a situation in which a vehicle with the present invention has been damaged at one wheel end by an IED, the operator may have the ability to still drive the vehicle to a safer location because the remaining wheel ends remained free to rotate.
In another embodiment of the present invention, similar functionality may be obtained with the combination of a pressure protection valve and a check valve, for example, a Bendix PR-4 valve and a SC-1 Single Check valve, both also available from Bendix Commercial Vehicle Systems LLC, installed in parallel to one another in the supply line between the wheel end's spring brake actuator and the upstream pressure modulator. During normal operation, pressure is supplied from the upstream pressure modulator through the pressure protection valve to the wheel end's spring brake actuator. During abnormal conditions, similar to the pilot-operated, non-graduated pneumatic control valve such as an SV-1 Synchro Valve, a sudden decrease in pressure from a ruptured line to the wheel end's spring brake actuator will result in the pressure protection valve automatically closing to shut off the flow from the upstream pressure modulator. However, because pressure protection valves typically do not have local exhaust ports, a check valve arranged in parallel with the pressure protection valve must be provided to provide a positive pressure relief path from the spring brake actuator during normal operations. By providing a pressure protection valve in parallel with a check valve, the arrangement ensures that in normal operation there remains a flow path which permits pressure to be removed from the spring brake actuator back through the pressure modulator (the flow-permitted direction of the check valve), while also ensuring that this normally-open flow path is shut when the downstream line is breached to prevent discharge of pressure from the upstream pressure modulator (the flow-blocked direction of the check valve).
In another embodiment, a velocity-sensing check valve may be placed in the line between the pressure modulator and the spring brake actuator at each wheel end. Such a valve is actuated to limit (i.e., meter) flow when the fluid medium passing therethrough reaches a predetermined velocity. With appropriate adjustment (“tuning”) of the valve actuating set point, the valve would permit normal pressure flow in normal operations (i.e., flow from the upstream pressure modulator to the spring brake actuator to release the parking brake, and flow back out of the spring brake actuator to set the parking brake), while still being ready to cut off high velocity flow resulting from rupture of the line to the spring brake actuator.
The identification of the foregoing flow control components is not intended as an exhaustive listing of flow control arrangements, as any other flow control device or combination of devices which provide the automatic damaged line shutoff functionality are within the scope of the present invention. In addition, the invention is not limited to damage affecting solely the lines between the flow control components and a parking brake actuator, but includes any damage at a wheel end which may result in the release of parking brake release pressure, such as damage to the parking brake release actuator portion of a wheel end's spring brake actuator.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.